We determined that the majority of cells produced using the current protocol were MGE-like progenitors or cortical interneurons, but we found that additional cell types were present in culture, including unventralized PAX6+ progenitors and LGE-like neurons. The cell types we observed may be unique to the conditions used, but single-cell resolution will provide insight into the development of effective protocols for generating specific populations, especially for therapeutic transplant purposes in which it is important to have an understanding of the cells present. Notably, there were no PVALB-expressing interneurons present in our cultures at any time point. Previous work has suggested that this population is sensitive to extrinsic factors such as activity and connectivity, and requires a protracted period of maturation (Cellerino et al., 1992; Nicholas et al., 2013). Likely we did not observe these cells because there were no excitatory neurons present in culture to provide input. Future studies connecting the transcriptomic identity of human MGE-like progenitors, with fate acquired after transplant into mouse cortex, will likely be required to determine which conditions are optimal for production of this cell type, as they are unlikely to reach maturity in vitro.
